Group III-V structures are useful for generating light in conventional light-emitting diode (LED) elements. Group III-V materials have also been proposed for use as three-dimensional structures such as an edge-emitting GaN disk-in-nanowire array electrically pumped laser as described in the article “Monolithic Electrically Injected Nanowire Array Edge-Emitting Laser on (001) Silicon” by Thomas Frost et al., Nano Lett. 2014, pages 4535 to 4541. In this structure, the nanowires are contacted at the top and bottom.
A silicon or germanium (Group IV material) integrated laser is described in US-A-2014/0175490 in which fins are combined with a waveguide. An array of vertical fins are provided in a laser cavity with a waveguide being formed on top of the fins for optical guiding and collection. Contacts are made on the sides of the fins.
However, when considering Group III-V materials, an epitaxial layer is prone to defects due to direct epitaxial growth of indium-phosphide/indium-gallium-arsenide (InP/InGaAs) on silicon. These defects include misfit dislocations (MDs), threading dislocations (TDs), stacking faults (SFs), and, point defects (PDs). Whilst MDs tend to be present at the interface with the substrate on which the epitaxial layer is grown, TDs extend as linear defects and SFs extend as planar defect from these points into the epitaxial layer, while PDs may be present throughout the body of the epitaxial layer. The presence of such defects makes it difficult to grow Group III-V materials which are suitable for laser applications.
It is also difficult to provide an optical cavity which has low parasitic losses with strong overlap of optical mode with an active gain medium, such as, an InGaAs active layer. Low loss feedback mechanisms are also required. Moreover, there are challenges with providing good carrier inversion in the optical gain layer, namely, free electron/hole injection when embedded in a forward biased p-i-n junction. Low series resistance is required for ohmic contacts to the p- and n-sides of the p-i-n junction which requires doping levels to be adjusted to avoid high optical loss. In addition, low parasitic losses in the laser cavity or waveguide need to be preserved so that there is a requirement for low optical overlap of contact layers with the optical mode of the fin structure.
Furthermore, efficient, low-loss coupling interfaces are needed between the laser cavity waveguide and passive (output) waveguides.